Vibration monitoring systems for aircraft engines usually comprise at least one measuring feeler, particularly an accelerometer connected to a measuring and detecting system. Each of the feelers is mounted at a particular place of the engine in order to measure one particularly interesting vibration, and each of the feelers is connected to an individual measuring system the output of which is operating a display unit. This display unit usually comprises a meter continuously indicating the level of vibration and an alarm lamp and/or bell for indicating alarm conditions to the crew. Such a system has a number of drawbacks. Whenever a feeler or a measuring or indicating unit is defectuous a proper monitoring of at least one typical vibration of the engine is no longer obtained. On the other hand if the defect of the feeler and/or measuring system results in an increase of the value indicated an alarm bell or lamp is operated, this forcing the crew to take measures, for instance to shut down the engine. There is no possibility to check the operating condition of the feeler and/or measuring and display system and thereby to distinguish between real alarm conditions in the engine and a defect of the feeler and/or measuring and display unit.